Artificial valves, both those used as prosthetic devices within the human and animal body and those used to control the flow of body fluids externally, have long been known to the medical profession. The prior art valves used primarily as prosthetic devices have taken many forms in an attempt to replicate the function of the natural valves which they replace. None of these prior art valves, however, has fully achieved the replication of a naturally occurring valve. Lack of success has resulted in part from problems with long term durability of the valve itself and from differences between actual and measured fluid flow area provided by the valve which has resulted in undesirable pressure gradients across the valve. In addition, prior art valves increase, rather than reduce or eliminate, the turbulence created by the passage of the fluid through and the subsequent closure of the valve. Such turbulence results in greatly increased hemolysis when the fluid flowing through the valve is blood.
Prior art prosthetic valves conform, for the most part, to three general types. The ball and cage valve, such as that described in U.S. Pat. No. 3,416,159, is illustrative of one type. The valve ball, usually constructed of silicone rubber, is susceptible to variance in shape, fractures of the ball surface and subsequent lipid infiltration into the ball. Additional complications which often accompany this type of valve include fibrin clot formation, which further interfers with the movement of the ball in the cage, and thrombosis. Because of these complications, the patient with this type of prosthesis must undergo continuous anticoagulant therapy. Moreover, since the ball portion of the valve must be located in the center of the cage to allow proper functioning of the valve, fluid flow through this type of valve cannot be central, as in a natural valve, but is directed through the valve around the periphery of the ball. The turbulence resulting from this, coupled with the turbulence created by closure of the valve, causes a higher level of hemolysis and blood cell damage than is desirable. The presence of the ball in the center of the blood vessel also imparts an unnatural radial component to the flow of blood within the vessel producing injury to the walls of the vessel against which the radial component is directed. Ball and cage valves are large in size and therefore can present difficulties in insertion. They have also been disturbing to some persons into whom they have been inserted because of the audible sound detectable upon closing. Attempts to correct the deficiencies of the ball and cage type valve by replacing the ball with a disc shaped element have generally been unsuccessful.
A second type of prior art valve, generally referred to as a tissue valve, is composed of a stent or mounting ring to which human or animal tissue has been attached in a form which approximates the flaps in a natural valve. However, the long-term durability of these valves is still a matter of concern. Construction of the tissue valve from cadaver material such as fascia lata results in a valve with low durability because of rapid tissue degradation and stiffening and has led to tissue dysfunction and subsequent valve immobility. While another variation of the tissue valve, the glutaraldehyde-fixed porcine xenograft, illustrated by U.S. Pat. No. 4,084,268 to Ionescu et al, has not been as prone to these problems, degradation and calcification of the tissue matrix resulting in valve dysfunction have been reported. Moreover, Johnson et al. in The Journal of Thoraic and Cardiovascular Surgery 75:599-605, 1978, concluded that functional stenosis or narrowing of the blood vessel was commonly encountered when the porcine xenograft was used to replace the aortic valve. Moreover, since these valves rely entirely on back pressure for closure, closure is accompanied by increased trauma to the blood cells and the greater likelihood of valve incompetence than encountered with a natural valve.
U.S. Pat. Nos. 3,197,788 to Segger and 3,736,598 to Bellhouse et al described prosthetic cardiac valves that are similar to tissue valves in their structure, but are made of flexible synthetic materials. Both valves disclosed in U.S. Pat. Nos. 3,197,788 and 3,736,598 imitate the natural valve by providing three cusps or flaps, which comprise the valve members, attached to a supporting ring. The valves open in response to the pressure exerted on them during systole and close in response to the back pressure exerted on the flaps during diastole. However, this closure is quick and somewhat traumatic and enhances the likelihood of increased turbulence and subsequent damage to the blood cells.
A third category of prosthetic valve is the disc type, which is generally formed of an annular base to which a disc-shaped valving member is secured, either by means of a magnetic hinge, as in U.S. Pat. No. 3,370,305 to Goott, by means of an eccentrically-placed stem, as in the Modified University of Capetown Prosthesis described by Ellis et al, in The Annals of Thoracic Surgery 23:26-31, 1977, or by means of a rod along which the disc can be displaced, as in U.S. Pat. No. 3,959,827 to Kaster. Although fluid flow through this type of valve is more centralized than through the ball and cage valve, increased hemolysis results from turbulence created by fluid flow through the valve. In addition, injury to the walls of the blood vessel beyond the valve results from the radial components imparted to the blood flow caused by the presence of the valve disc in the blood stream. Moreover, use of this valve necessitates the institution of long-term anticoagulant therapy because of the high level of thrombogenesis which accompanies the use of the valve. As reported in The Annals of Thoracic Surgery 23:26-31, 1977, Ellis et al discontinued use of one type of tilting disc prosthesis, in part because of the excessive incidence of thromboembolism and in part because the effective valve orifice area after the valve had been in place was usually considerably less than the measured orifice area before insertion.
In U.S. Pat. No. 3,959,827, Kaster discloses one embodiment of a disc type valve in which the closing of the valve is assisted by means of a permanent magnet located in the disc valving member. Although this permits a smoother closing than is possible without the magnet, the disc valve disclosed in U.S. Pat. No. 3,959,827 still suffers from the other drawbacks generally common to disc type valves. Forman et al reported in The Journal of Thoracic and Cardiovascular Surgery 75:595-598, 1978, that they no longer recommended the use of a tilting disc valve similar to the one disclosed in U.S. Pat. No. 3,959,827, in large part because of a high incidence of embolism and valvular thrombosis, but also because the effective orifice area of the valve was less than the actual orifice area. In addition, they found that this type of valve demonstrated no clear hemodynamic advantage over other available prostheses.
Valves used for medical purposes in which valve closing is magnetically assisted are disclosed in U.S. Pat. Nos. 3,233,610 to Wade, 3,495,620 to Raimondi et al and 3,926,175 to Allen et al. The valves described in U.S. Pat. Nos. 3,233,610 and 3,495,620 provide fluid flow orifices through only a portion of the valve diameter, thus resulting in a slow fluid flow area in relation to valve diameter. The magnetically actuated valve disclosed in U.S. Pat. No. 3,926,175 is incapable of operating in response to fluid pressure within a body vessel and is thus unable to operate automatically to cause unidirectional flow of a pulsatory fluid.